Field of the Invention
The invention relates to a method for monitoring a transmitter. Here, the transmitter comprises at least one measurement unit and one transmission unit. The measurement unit and the transmission unit are interconnected at least for transmission of signals. The measurement unit is used to determine at least one measurement quantity and to produce a measurement signal which is dependent on it. The transmission unit receives the measurement signal from the measurement unit and is used to transfer an output signal to at least one signal transmission element, based on the measurement signal. The invention also relates to a corresponding transmitter which comprises at least one measurement unit and one transmission unit.
Description of Related Art
Transmitters—alternative names are transducers or field devices—of the aforementioned type have been known for a long time and are used quite generally to detect the primary sensor signal which originates from a sensor unit or a sensor element and to convert it into a generally standardized signal as an electrical output signal so that it can be used for example, by a higher-level process monitoring unit or control unit.
The term transmitter should be understood as not being limiting in any way. The sensor unit can be used especially for measurement of physical or chemical measurement quantities, for example, flow rate, mass flow rate, pressure, temperature, fill level, pH value, etc. This transmitter can be divided at least into two parts for the general examination: into a measurement unit which is used for the measurement in itself and which provides a measurement signal originating from the measurement, and into a transmission unit which receives the measurement signal and transfers it, for example, via a field bus to a higher-level unit. Thus, one part of the transmitter performs the task of actual measurement and the other part of the transmitter communicates the measurement value which has been obtained in the measurement, or optionally, the acquired measurement values to other units.
Depending on the application, transmitters must satisfy different safety requirements. Thus, in particular, possible error sources must be recognized, and optionally, there must be safety measures. For errors, critical regions are, for example, the measurement itself, the signal processing or data processing in the transmitter and also the communication with, for example, the units which are at a higher level than the transmitter. In order to satisfy the respective safety requirements, for example, the SIL (Safety Integrity Level) standard which is important in process automation, there must be, for example, redundancy or diversity for the transmitters. Redundancy, here, means a doubled or multiple layout of safety-relevant components. Diversity means that the hardware components or software programs used originate from different manufacturers or are of different type. Both redundant and also diverse configurations are generally complex and/or cost-intensive.
For example, European Patent Application EP 1 466 308 B1 and corresponding U.S. Pat. Nos. 7,539,600 and 7,139,683 disclose a sensor arrangement with a measurement detector which generates a raw signal, and with an output stage which outputs an output signal. Between the measurement detector and the output stage, transmission and conversion of the raw signal into the output signal occur. An additional monitoring unit generates an auxiliary signal from the raw signal and compares it to the output signal in order to signal deviations beyond a predetermined framework. It is disadvantageous that a second unit must be provided for processing of the raw signal. At the same time, only the signal path within the transmitter is monitored in this way.
A quantity which is relevant for assessing a transmitter with respect to safety is the safe failure fraction (SFF) which indicates how large the portion of the safe error in the altogether possible errors is. A safe error is an error which is relevant to safety and which however either is recognized or transfers the transmitter into a safe state.